A single-end-sealed tungsten halogen lamp 17 as shown in FIG. 5 is known as a conventional tungsten halogen lamp (Japanese Patent Application No. (Tokkai Sho) 57-74963). In the tungsten halogen lamp 17, an infrared reflecting film 16 is formed on the surface of a straight-tube-shaped arc tube 15, in which a filament coil 14 is located, by alternately dipping the arc tube 15 in a solution for forming a TiO.sub.2 film and a solution for forming a SiO.sub.2 film.
In the conventional tungsten halogen lamp, gaps 18 that are not hermetically sealed occur between the quartz glass of a sealing portion 19 and metal foils 20 and outer leads 21, along parts of the metal foils 20 of molybdenum sealed in the sealing portion 19, and along the outer leads 21 having one end connected to the metal foils 20 and the other end led out of the sealing portion 19.
When the gaps 18 are present, air enters into the sealing portion 19 through the gaps 18, so that the metal foils 20 in the sealing portion 19 are oxidized during the lamp life. Therefore, leaks and cracks are eventually caused in the sealing portion 19, shortening the lamp life. In addition, the lamp efficiency of the tungsten halogen lamp increases only by about 7% by forming the infrared reflecting film 16.
Another conventional tungsten halogen lamp as shown in FIG. 6 is known (U.S. Pat. Nos 5,045,748 and 5,138,219). The tungsten halogen lamp comprises a double-end-sealed elliptical arc tube 22 of fused quartz in an outer tube 24. An infrared reflecting film 23 is formed on the surface of the arc tube 22 by a CVD technique (chemical vapor deposition technique). With the CVD technique, the arc tube 22 is put into an evacuated furnace, and tantalum (Ta) and silicon (Si) atmospheres are created alternately in the furnace.
The luminous efficiency of this conventional tungsten halogen lamp increases by about 50% because of the infrared reflecting film 23 and the elliptical arc tube 22. However, since the tungsten halogen lamp has a double-tube structure in which the arc tube 22 is held in the outer tube 24, the structure is complicated and involves a high cost.
In order to solve the above problems, it is an object of the present invention to provide a tungsten halogen lamp that has a long life and a high efficiency and is inexpensive, and a method for manufacturing the same, by preventing the oxidation of the metal foils.